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The purpose of this study was to compare the effect of fresh mineral trioxide aggregate (MTA) with set MTA on hard-tissue healing after periradicular surgery. The root canals of 24 mandibular premolars in four 2-yr-old beagle dogs were filled with MTA. Two weeks later the root ends of half of the samples were surgically exposed and resected to the set MTA within the canals. After exposing and resecting the other 12 root ends, they were prepared with ultrasonic instrumentation and preparations were filled with fresh MTA. After 4 months, the animals were killed. Hard-tissue healing was analyzed histomorphometrically.The results indicated that although freshly placed MTA resulted in a significantly higher incidence of cementum formation (12 of 12 versus 8 of 12, p = 0.028), there is no significant difference in the quantity of cementum or osseous healing associated with freshly placed or set MTA when used as root-end-filling material.